Some conventional physical quantity detectors detect pressure changes within a vehicle door by plural acoustic microphones and detect collision against a vehicle side based on a signal that is wave-shaped by a band pass filter (e.g., JP-5-66228A).
Moreover, other conventional detectors detect collision regardless of a change in atmospheric pressure by compensating and controlling the sensitivity of a pressure sensor that detects pressure according to a change in the atmospheric pressure (e.g., U.S. Pat. No. 5,974,892 and JP-11-500218A).
However, as described in JP-5-66228A, in an attempt to detect collision against a vehicle side by detecting an abrupt pressure change within a vehicle door, since pressure within the door is changed by not only collision but also the atmospheric pressure, detection precision of collision would vary depending on changes in the atmospheric pressure.
FIG. 8 shows two pressure changes within a vehicle door caused by collision against the door. One represents a case where an atmosphere pressure is as high as Pb0, and the other presents a case where an atmosphere pressure is as low as Pa0. Pressure P within a vehicle door begins to rise from an atmospheric pressure before collision when the collision occurs at time t0, and falls back to the atmospheric pressure at time t2 after becoming maximum at a certain time (time t1). A change quantity of pressure within the door caused by the collision is smaller when the atmospheric pressure is low than when the atmospheric pressure is high. Specifically, a change quantity ΔPa of pressure within the door in the case of atmospheric pressure Pa0 is smaller than a change quantity ΔPb of pressure within the door in the case of atmospheric pressure Pb0.
When pressure within a vehicle door is P, an atmospheric pressure is P0, a pressure change caused by collision is ΔP, a space volume within the door before collision is V0, and a space volume change caused by collision is ΔV a change ΔP of the pressure P within the door at collision is expressed by equation 1, based on the equation of state for the ideal gas.ΔP=−(ΔV/V0)×P0   [Equation 1]
It will be understood from the equation 1 that a pressure change ΔP caused by collision is proportional to the atmospheric pressure P0. Therefore, at a high altitude, the atmospheric pressure becomes low and hence a pressure change ΔP becomes small, so that detection precision of collision decreases.
That is, the atmospheric pressure P0 is considered as an average physical quantity of a detection target, a pressure change ΔP from the atmospheric pressure P0 is considered as a change quantity from the average physical quantity, and a detection precision of a physical quantity of the detection target decreases due to a change in the average physical quantity of the detection target.
The detector disclosed in U.S. Pat. No. 5,974,892 therefore includes a circuit for compensating and controlling sensor sensitivity to reduce influence by a change in atmospheric pressure.